Power density is always a key performance metric of a power supply circuit such as AC-DC, DC-AC and DC-DC power converters to provide the smallest possible physical size for a given output power specification. Resonant power converter topologies are well-known types of DC-DC/switched mode power supplies or converters (SMPS) in the art. Resonant power converters are particularly useful for high switching frequencies such as above 1 MHz where switching losses of standard SMPS topologies (Buck, Boost etc.) tend to be unacceptable for conversion efficiency reasons. High switching frequencies are generally desirable because of the resulting decrease of the electrical and physical size of circuit components of the power converter like inductors and capacitors. The smaller components allow increase of the power density of the SMPS. In a resonant power converter an input “chopper” semiconductor switch (often MOSFET or IGBT) of the standard SMPS is replaced with a “resonant” semiconductor switch. The resonant semiconductor switch relies on the resonances of circuit capacitances and inductances to shape the waveform of either the current or the voltage across the switching element such that, when switching takes place, there is no current through or voltage across the switching element. Hence power dissipation is largely eliminated in at least some of the intrinsic capacitances of the input switching element such that a dramatic increase of the switching frequency becomes feasible for example to values above 10 MHz. This concept is known in the art under designations like zero voltage and/or current switching (ZVS and/or ZCS) operation. Commonly used switched mode power converters operating under ZVS and/or ZCS are often described as class E, class F or class DE inverters or power converters.
However, fast and accurate control of the output voltage of the resonant power converter remains a challenge. Prior art power converters described in the references below propose to utilize a self-oscillating feedback loop around the input switching element and driven by the intrinsic or inherent drain-to-source capacitance of a MOSFET switch in combination with a variable series inductance coupled to the gate terminal of the MOSFET switch.
U.S. Pat. No. 4,605,999 discloses a self-oscillating power converter comprising a self-oscillating inverter circuit build around a single MOSFET switch. The inherent drain-to-source capacitance of the MOSFET switch supplies a feedback path sufficient to sustain self-oscillation of the inverter circuit if the frequency of operation is sufficiently high. The power converter is voltage regulated by a feedback loop deriving the control signal from a DC output voltage of the converter and applying the control signal to a variable inductance network comprising an inductor and a pair of non-linear capacitances.
U.S. Pat. No. 5,430,632 discloses a self-oscillating power converter utilizing a pair of MOSFET transistor switches in a half bridge configuration wherein the junction of the two MOSFET transistors is coupled to a reactive network which in turn is connected to an output rectifier. Intrinsic gate-to-drain inter-electrode capacitances of the switching transistors serve as the sole means of sustaining oscillations. Oscillations are initiated at the gate-to-source terminals of the MOSFET transistor switches by a start-up circuit. The frequency of oscillation is determined by the gate-to-source capacitance of the MOSFET transistor switches and the inductance of an isolated gate drive transformer. The frequency of oscillation is controlled by varying inductance of the isolated gate drive transformer coupled to the gate terminals of the MOSFET transistor switches through a pair of control windings.
However, the possible regulation range of adjustable inductances and/or capacitances tend to be very narrow due to physical component limitations and the accuracy may also be limited. Furthermore, adjustable inductances and/or capacitances are difficult to integrate on semiconductor substrates or on ordinary circuit carriers like printed circuit boards. Finally, the maximum regulation speed of the inductance or capacitance may be limited due to the reactive nature of the component leading to an undesirable limitation of the speed of the regulation of the converter output voltage. This is of course particularly undesirable in view of the advantages of moving to higher converter switching frequencies for the reasons discussed above.
Consequently, it would be advantageous to provide a control mechanism for the oscillation frequency that eliminates the need of variable reactive components like inductors and capacitors such that the converter output voltage can be controlled by appropriately controlling a level of a circuit voltage or circuit current for example in the form of an adjustable bias voltage.